On 11 March 2011, a major nuclear accident began at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima, Japan. The main reason was the Tōhoku earthquake and tsunami, which caused the power supply to fail and damaged most of the power plant’s backup energy sources. This made it difficult to cool the reactors after they shut down, leading to the release of radioactive materials into the environment. Experts from the United Nations Scientific Committee on the Effects of Atomic Radiation say this was the worst nuclear incident since the Chernobyl disaster.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation, no health problems among Fukushima residents have been proven to be directly caused by radiation from the accident. Insurance payments were made for one death from lung cancer, but this does not show that radiation caused the cancer. Six other people are reported to have developed cancer or leukemia. Two workers were hospitalized due to radiation burns, and others suffered physical injuries from the accident.
After the accident, at least 164,000 people from the surrounding area were displaced, either by choice or because they were ordered to leave. This caused at least 51 deaths and increased stress and fear about radiation risks. Some people say the evacuation caused more harm than it prevented. Ten years later, over 41,000 people from Fukushima were still living as evacuees.
Studies found problems with safety and planning, including poor risk assessments and evacuation plans. Disputes remain about how to handle treated water used to cool the reactor, which has led to protests in nearby countries.
The cost of cleaning up the radioactive waste and paying compensation to victims was estimated by Japan’s trade ministry in November 2016 to be 20 trillion yen (about 180 billion US dollars).
Background
The Fukushima Daiichi Nuclear Power Plant had six General Electric (GE) light water boiling water reactors (BWRs). Unit 1 was a GE type 3 BWR. Units 2–5 were type 4. Unit 6 was a type 5.
On 11 March 2011, during the Tōhoku earthquake, units 1–3 were operating. However, the spent fuel pools of all units still needed cooling.
Many parts inside the reactors and the fuel assembly cladding are made from a zirconium alloy (Zircaloy) because it has a low neutron cross section. At normal operating temperatures (~300 °C (572 °F)), Zircaloy does not react. However, above 1,200 °C (2,190 °F), Zircaloy can react with steam to form hydrogen gas or with uranium dioxide to form uranium metal. Both reactions release heat. When combined with the heat from boron carbide reacting with stainless steel, these reactions can cause the reactor to overheat.
In emergencies, reactor pressure vessels (RPV) are automatically disconnected from turbines and the main condenser and instead connected to a secondary condenser system. This system cools the reactor without needing external power or generators. The isolation condenser (IC) system uses a closed coolant loop from the pressure vessel with a heat exchanger in a dedicated condenser tank. Steam is pushed into the heat exchanger by reactor pressure, and the cooled water is returned to the vessel by gravity. Each reactor was originally designed with two backup ICs, each capable of cooling the reactor for at least 8 hours. However, the IC system could cool the reactor too quickly after shutdown, causing thermal stress. To prevent this, operators were required to manually control the condenser loop using electrically operated valves.
After Unit 1 was built, later units were designed with new open-cycle reactor core isolation cooling (RCIC) systems. These systems used steam from the reactor to power a turbine, which operated a pump to inject water into the pressure vessel from an external storage tank. This helped maintain water levels in the reactor. The system was designed to operate for at least 4 hours or until coolant ran out or the system failed. If the storage tank ran out, the system could switch to a closed-loop system, drawing coolant from the suppression chamber (SC) instead. Although the system did not need external power (except for steam), direct current (DC) was required to control it remotely, and alternating current (AC) was needed to operate isolation valves.
If backup on-site power was damaged or not enough to last until off-site power could be restored, cooling systems could not be relied on. In such cases, operators would vent the reactor vessel and primary containment using electrically or pneumatically operated valves to lower pressure. This allowed low-pressure water injection into the reactor using the fire protection system to replace water lost to evaporation.
Station operators switched reactor control to off-site power for shutdown, but the system was damaged by the earthquake. Emergency diesel generators (EDG) then automatically started to provide AC power. Two EDGs were available for units 1–5 and three for unit 6. Of the 13 EDGs, 10 were water-cooled and placed in basements about 7–8 meters below ground level. The coolant water for these EDGs was supplied by seawater pumps on the shoreline, which also provided water for the main condenser. These components were not protected by buildings, only by the seawall. The other three EDGs were air-cooled and connected to units 2, 4, and 6. The air-cooled EDGs for units 2 and 4 were placed on the ground floor of the spent fuel building, but their switches and components were in the basement. The third air-cooled EDG was in a separate building inland at higher ground. Although EDGs were intended for specific reactors, switchable connections between units (1 and 2, 3 and 4, 5 and 6) allowed reactors to share EDGs if needed.
The power station also had backup DC batteries charged by AC power at all times. These batteries could power the station for about 8 hours without EDGs. In units 1, 2, and 4, the batteries were located in the basements with the EDGs. In units 3, 5, and 6, the batteries were placed in the turbine building above ground level.
The units and central storage facility contained the following numbers of fuel assemblies:
The original design for the reactors was based on a zero-point ground acceleration of 250 Gal and a static acceleration of 470 Gal, using data from the 1952 Kern County earthquake (140 Gal, 0.18 g, 1.4 m/s², 4.6 ft/s²). After the 1978 Miyagi earthquake, when ground acceleration reached 122 Gal (0.125 g, 1.22 m/s², 4.0 ft/s²) for 30 seconds, no damage was found to critical reactor parts. In 2006, the reactor designs were updated to withstand accelerations up to 450 Gal.
In emergencies, operators planned to pump water into reactors to keep them cool. This process would create steam, which would not be highly radioactive because the fuel would remain inside the primary containment vessel. To avoid high-pressure explosions, operators would manually release the steam through venting valves.
Accident
A magnitude 9.0 earthquake occurred on Friday, 11 March 2011, at 2:46 p.m. The earthquake’s epicenter was located off the east coast of the Tōhoku region. The earthquake caused ground acceleration measurements of 560, 520, and 560 Gal at units 2, 3, and 5, respectively. These values exceeded the design limits of 450 Gal, 450 Gal, and 460 Gal for continued operation in those units. However, the measurements were within the design limits for unit 6.
When the earthquake was detected, all three operating reactors (units 1, 2, and 3) automatically shut down. Because the earthquake caused expected damage to the power grid and the switch station, the power station activated emergency diesel generators (EDGs), isolated the reactors from the primary coolant loops, and started emergency shutdown cooling systems.
The largest tsunami wave was 13 to 14 meters (43 to 46 feet) high. It hit the area about 50 minutes after the earthquake, overtopping a seawall and flooding the plant’s ground level, which was 10 meters (33 feet) above sea level. The tsunami first damaged seawater pumps along the shoreline and 10 of the plant’s 13 cooling systems for the EDGs. It then flooded turbine and reactor buildings, damaging EDGs and electrical components on the ground or basement levels at approximately 3:41 p.m. Switching stations that provided power from EDGs located on a hillside also failed when the buildings housing them flooded. One air-cooled EDG from unit 6 was not affected by the flooding and continued to operate. The DC batteries for units 1, 2, and 4 became inoperable shortly after the flooding.
As a result, units 1 through 5 lost AC power, and units 1, 2, and 4 lost DC power. In response, operators assumed a loss of coolant in units 1 and 2 and planned to vent the primary containment and inject water into the reactor vessels using firefighting equipment. Tokyo Electric Power Company (TEPCO), the utility operator and owner, notified authorities of a "first-level emergency."
Two workers were killed by the tsunami.
The isolation condenser (IC) was working before the tsunami, but the DC-operated control valve outside the primary containment was closed to prevent thermal stress on reactor components. Some control room indicators stopped functioning, and operators correctly assumed a loss of coolant (LOC). At 6:18 p.m. on 11 March, a few hours after the tsunami, operators tried to manually open the IC control valve, but the IC failed to function, suggesting the isolation valves were closed. Although the valves were kept open during IC operation, the loss of DC power in unit 1 (which occurred shortly before the loss of AC power) automatically closed the AC-powered isolation valves to prevent uncontrolled cooling or a potential LOC. Operators did not know this status but correctly interpreted the IC system’s failure and manually closed the control valves. Plant operators continued to attempt to restart the IC in the following hours and days, but it did not work.
Plant operators then tried to use fire protection (FP) equipment, powered by a diesel-driven fire pump (DDFP), to inject water into the reactor vessel. However, the reactor pressure had already risen far above the DDFP’s limit. Operators also detected high radiation levels in the secondary confinement structure, indicating damage to the reactor core, and found that the primary containment vessel (PCV) pressure (0.6 MPa) exceeded design specifications (0.528 MPa). In response, operators planned to lower the PCV pressure by venting. The PCV reached its maximum pressure of 0.84 MPa at 2:30 a.m. on 12 March, after which it stabilized around 0.8 MPa. The pressure drop was due to an uncontrolled vent via an unknown pathway. The plant was notified that Okuma town completed its evacuation at 9:02 a.m. on 12 March. Staff then began controlled venting, which was completed later that afternoon at 2:00 p.m.
At the same time, pressure in the reactor vessel decreased to match the PCV, and workers prepared to inject water into the reactor vessel using the DDFP once pressure dropped below 0.8 MPa. However, the DDFP was inoperable, so a fire truck had to be connected to the FP system. This process took about 4 hours because the FP injection port was hidden under debris. The next morning (12 March, 4:00 a.m.), approximately 12 hours after the power loss, freshwater injection into the reactor vessel began, later replaced by a water line at 9:15 a.m. connecting directly from the water storage tank to the injection port for continuous operation (the fire truck had to be periodically refilled). This continued into the afternoon until the freshwater tank was nearly empty. In response, injection stopped at 2:53 p.m., and seawater injection began using water collected in a nearby valve pit (the only other water source). Power was restored to units 1 (and 2) using a mobile generator at 3:30 p.m. on 12 March.
At 3:36 p.m., a hydrogen explosion damaged the secondary confinement structure (the RB). Workers evacuated shortly after the explosion. Debris from the explosion damaged the mobile emergency power generator and seawater injection lines. The seawater injection lines were repaired and put back into operation at 7:04 p.m. until the valve pit was nearly empty at 1:10 a.m. on the 14th. Seawater injection was temporarily stopped to refill the valve pit with seawater using emergency service and JSDF vehicles. However, the process was interrupted by another explosion in unit 3 RB at 11:01 a.m., which damaged water lines and prompted another evacuation. Seawater injection into unit 1 did not resume until that evening, after 18 hours without cooling.
Analysis in November 2011 suggested that the extended period without cooling caused the fuel in unit 1 to melt. Most of the melted fuel likely escaped the reactor pressure vessel (RPV) and embedded itself into the concrete at the base of the primary containment vessel (PC
Consequences
During the early hours of the accident, a blackout at the power station and uncertainty about the cooling systems of units 1 and 2 led to an order to evacuate people within a 2 km radius, affecting 1,900 residents, at 20:50. However, because coordination with the national government was difficult, a new order was issued at 21:23 to evacuate 6,000 residents within a 3 km radius and to keep 45,000 residents within a 10 km radius in place. The evacuation area was later expanded to 10 km at 5:44 and then to 20 km at 18:25. The size of these zones was decided without clear reasons by government officials, not by nuclear experts. Communication between different groups was unclear, and local governments often learned about evacuation orders through televised news. Citizens were informed by radio, trucks with loudspeakers, and visits to homes. Many local governments ordered evacuations on their own before national orders because they could not contact higher authorities. By the time the 3 km order was issued, most people in that area had already left.
Because of overlapping evacuation orders, many residents moved to areas that would later become evacuation zones. This caused many people to move multiple times until they reached a place outside the final 20 km zone. About 20% of people who lived within the initial 2 km area had to leave more than six times.
A 30 km shelter-in-place order was announced on the 15th, but some areas had already decided to evacuate. A voluntary evacuation recommendation was given on the 25th, but most people had already left the 30 km zone by then. The shelter-in-place order ended on April 22, but the evacuation recommendation stayed in place.
Of about 2,220 patients and elderly people in hospitals and nursing homes within the 20 km zone, 51 deaths were linked to the evacuation. Dr. Arifumi Hasegawa, a radiation medicine expert, suggested that hypothermia, worsening health conditions, and dehydration may have caused these deaths. The lack of medical care before, during, and after the evacuation was seen as the main reason for these deaths. The Fukushima accident showed serious problems with evacuating hospitals and nursing homes.
One worker at the power plant died of lung cancer four years after the accident, having been exposed to 74 mSv of radiation. However, Geraldine Thomas said the chance that the cancer was caused by radiation was extremely low.
The Japanese public believed the government and TEPCO did not share enough information about the accident early on. Experts who explained the accident in simple terms were not from the government or TEPCO, but from Masashi Gotō, a retired engineer who worked for Toshiba, the company that built four of the six reactors. Gotō gave press briefings starting March 14, 2011.
There were times when data about the accident was not handled properly. The Ministry of Education, Culture, Sports, Science, and Technology (MEXT) sent data from the SPEEDI network only to the Fukushima prefectural government and was later criticized for delaying information to the U.S. military. The U.S. military created a detailed map using planes and shared it with the Ministry of Economy, Trade, and Industry (METI) on March 18 and MEXT two days later. No new evacuation plans were made a week after the accident. The data was not shared with the Nuclear Safety Commission but was made public by the U.S. on March 23.
TEPCO officials were told not to use the phrase "core meltdown" to hide the fact until they officially acknowledged it two months later.
The Japanese government did not keep records of important meetings during the crisis. Emails from the Nuclear and Industrial Safety Agency to the Fukushima prefectural government, including evacuation and health advisories from March 12, 23:54, to March 16, 09:00, went unread and were deleted.
In January 2015, about 119,000 residents were still displaced due to the accident, with the number reaching 164,000 in June 2012. If people had stayed in place instead of evacuating, the loss of life would have been much smaller.
In the former Soviet Union, many people who had little radiation exposure after the Chernobyl accident experienced extreme fear of radiation. They developed health problems, including radiophobia and increased alcohol use. Shunichi Yamashita, a Japanese radiation expert, noted this.
A 2012 survey by the Iitate local government found that about 1,743 evacuees in the zone reported growing frustration, instability, and difficulty returning to their lives. Sixty percent said their health and their families’ health had worsened after leaving, and 39.9% felt more irritated than before the accident.
Stress can cause physical issues, such as poor eating habits, lack of exercise, and sleep problems. Survivors who lost homes, villages, or family members often faced mental and physical challenges. Much of the stress came from not having enough information and from being relocated.
A 2014 review of 48 studies found that many Fukushima residents experienced fear linked to depression, anxiety, sleep problems, post-traumatic stress, and stress among nuclear plant workers. The psychological stress among evacuees was five times higher than the average in Japan. An increase in childhood obesity was also linked to advice for children to stay indoors instead of playing outside.
Before the accident, over 25% of Japan’s electricity came from nuclear power, and the country aimed to reduce greenhouse gas emissions by 25% below 1990 levels by 2020. This plan included increasing nuclear power’s share from 30% to 50%. However, after the accident, the goal was changed to a 5.2% increase in emissions by 2020, with a focus on reducing nuclear power and using more renewable energy. Nuclear energy’s contribution dropped to less than 1% after the accident, and all reactors were shut down by 2013. This led to a rise in fossil fuel use, reaching about 94% by 2015, the highest among IEA members. The increase in fossil fuel imports caused a trade deficit that lasted for years.
In the immediate aftermath, nine prefectures served by TEPCO faced power shortages. The government asked major companies to reduce electricity use.
Investigations
Three studies about the accident showed that the disaster was caused by human actions and was linked to a system where regulators prioritized the nuclear industry over public safety. This system involved a "network of corruption, collusion, and nepotism." A report by The New York Times explained that Japan’s nuclear regulatory system often supported the nuclear industry because of a practice called amakudari ("descent from heaven"), where senior regulators took high-paying jobs at companies they had previously overseen.
In August 2011, several top energy officials were removed from their positions by the Japanese government. These included the Vice-Minister for Economy, Trade and Industry; the head of the Nuclear and Industrial Safety Agency; and the head of the Agency for Natural Resources and Energy.
In 2016, three former TEPCO executives—chairman Tsunehisa Katsumata and two vice presidents—were charged with being responsible for deaths and injuries caused by their actions. They denied the charges, and in September 2019, the court agreed with their denial.
The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) was the first independent commission created by Japan’s National Diet in the 66-year history of the country’s constitutional government. The commission’s chairman stated that the disaster was predictable and could have been prevented. The report found that the government and TEPCO did not feel responsible for protecting people. It said they "effectively betrayed the nation’s right to be safe from nuclear accidents." The commission noted that the disaster had unique features tied to Japanese cultural traditions, such as obedience, a tendency not to question authority, and a focus on group harmony.
The commission acknowledged that residents affected by the disaster were still struggling with serious problems, including health risks from radiation, displacement, broken families, disrupted lives, and environmental damage.
The Investigation Committee on the Accident at the Fukushima Nuclear Power Stations (ICANPS) was formed to find the causes of the disaster and suggest policies to reduce harm and prevent similar events. The 10-member panel, appointed by the government, included scholars, journalists, lawyers, and engineers. It was supported by public prosecutors and government experts and released a 448-page report on July 23, 2012.
The panel’s report criticized Japan’s weak legal system for managing nuclear crises,混乱 in crisis leadership by the government and TEPCO, and possible overreach by the office of Prime Minister Naoto Kan during the early stages of the disaster. It concluded that a culture of carelessness about nuclear safety and poor crisis management led to the accident.
Remediation and recovery
To reduce people's fears, the government created an order to clean up more than 100 areas where radiation levels were higher than one millisievert per year. This level is much lower than what is needed to protect health. The government also tried to help people understand the effects of radiation and how much radiation people are usually exposed to.
In 2018, tours to visit the area where the accident happened began. In September 2020, a museum called The Great East Japan Earthquake and Nuclear Disaster Memorial Museum opened in the town of Futaba, near the power plant. The museum shows items and videos about the earthquake and nuclear accident. To help visitors from other countries, the museum provides information in English, Chinese, and Korean.
TEPCO plans to remove the remaining nuclear fuel from the power plant. In December 2014, TEPCO removed 1,535 fuel assemblies from the Unit 4 spent fuel pool. In February 2021, TEPCO removed 566 fuel assemblies from the Unit 3 spent fuel pool. TEPCO plans to remove all fuel rods from the spent fuel pools of Units 1, 2, 5, and 6 by 2037. It also plans to remove the remaining molten fuel debris from the reactor containments of Units 1, 2, and 3 by about 2050. Plant managers estimate that the cleanup and decommissioning process will take 30 to 40 years from the time of the accident.
As of 2013, about 400 metric tons of cooling water was pumped into the reactors each day. Another 400 metric tons of groundwater flowed into the structure each day. About 800 metric tons of water was removed daily for treatment. Half of this water was reused for cooling, and the other half was stored in tanks. After treatment, the contaminated water is released into the Pacific Ocean because it is not possible to remove tritium, a type of radioactive material. TEPCO built an underground ice wall to stop groundwater from entering the reactor buildings. A $300 million cooling facility freezes the ground to a depth of 30 meters. By 2019, the amount of contaminated water generated each day had dropped to 170 metric tons.
In February 2014, NHK reported that TEPCO was reviewing its radioactivity data after finding higher levels of radioactivity than previously reported. In July 2013, groundwater collected had 5 MBq of strontium per liter, not the 0.9 MBq that was first reported.
On September 10, 2015, floodwaters from Typhoon Etau caused mass evacuations in Japan and overwhelmed the drainage pumps at the power plant. Hundreds of metric tons of radioactive water entered the ocean. Plastic bags filled with contaminated soil and grass were also swept away by the floodwaters.
By October 2019, 1.17 million cubic meters of contaminated water was stored at the plant. A purification system removes most radioactive materials except tritium to levels allowed by Japanese regulations for discharge into the sea. By December 2019, 28% of the water met the required standards, while 72% needed further purification. Tritium cannot be removed from the water. At that time, the total amount of tritium in the water was about 856 terabecquerels, with an average concentration of 0.73 MBq per liter.
A 2020 committee set up by the Japanese government concluded that the purified water should be released into the sea or evaporated into the atmosphere. The committee estimated that releasing all the water into the sea over one year would expose local people to a radiation dose of 0.81 microsieverts, while evaporation would cause 1.2 microsieverts. For comparison, Japanese people receive about 2,100 microsieverts per year from natural radiation. The IAEA (International Atomic Energy Agency) said the method used to calculate radiation doses was appropriate. The IAEA also recommended that a decision on how to dispose of the water must be made quickly. Although the radiation doses are very small, the Japanese committee was worried that releasing the water might harm the reputation of the region, especially the fishing industry and tourism.
In 2021, Japan's Nuclear Regulation Authority warned that some of the 3,373 waste storage containers for radioactive slurry were degrading faster than expected. Moving the slurry to new containers was time-consuming, creating an urgent problem.
Tanks used to store water were expected to be filled by 2023. In July 2022, Japan's Nuclear Regulation Authority approved the release of treated water into the sea. Japan said the water is safe, and many scientists agreed. The decision came weeks after the UN's nuclear watchdog approved the plan. However, critics said more studies are needed, and the release should be stopped. In August 2022, Japan began releasing treated wastewater into the Pacific Ocean, causing protests in the region and retaliation from China, which blocked all imports of Japanese seafood. The plan was to release all the water over 30 years. A US State Department spokesperson supported the decision. South Korea's foreign minister and activists from Japan and South Korea protested the announcement. In April 2023, fishers and activists held protests in front of the Japanese embassy in the Philippines against the planned release of 1.3 million tons of treated water into the Pacific Ocean.
Xiaoqi Zhou, an environmental sciences scholar, found that more than 60 radioactive substances were detected in the Fukushima nuclear wastewater, including Ru, Co, Sr, Cs, and I. Some
Prior warning
On 5 July 2012, the NAIIC concluded that the causes of the accident could have been predicted, and that TEPCO did not follow basic safety rules, such as assessing risks, planning for damage control, and creating evacuation plans. Three months after the accident, during a meeting in Vienna, Austria, the IAEA said the Japanese government’s oversight was not enough, noting that the Ministry of Economy, Trade and Industry had a conflict of interest because it was responsible for both regulating and promoting nuclear power. On 12 October 2012, TEPCO admitted it did not take needed safety steps because it feared lawsuits or protests. Increased distrust in the government led to protests through street demonstrations, books, and films. In the Tohoku region, individuals, groups, and non-profits also worked to share radiation data in new ways to better inform the public about contamination and health risks.
In 1991, the U.S. Nuclear Regulatory Commission warned about the risk of losing emergency power. In 2004, the Nuclear and Industrial Safety Agency mentioned this report but did not take steps to reduce the risk. In 2000, a TEPCO report suggested safety steps to prevent flooding from seawater, based on a possible 15-meter (49 ft) tsunami. TEPCO did not act because it worried about causing public concern. In 2002, the government’s earthquake research team estimated a 15.7-meter (52 ft) tsunami could hit the power plant. These findings were supported by the cabinet office, which said TEPCO’s 5.6-meter (18 ft) estimate was too low. In 2008, a TEPCO study showed the need to improve flood protection, using the 15.7-meter (52 ft) estimate from 2002.
In 2009, the Active Fault and Earthquake Research Center asked TEPCO and the Nuclear and Industrial Safety Agency to update their tsunami height assumptions, based on research about the 869 Sanriku earthquake. However, this was not taken seriously at the time. On 30 October 1991, an emergency diesel generator in unit 1 failed due to a coolant leak, as reported by former employees in 2011. A 2011 TEPCO report said water entered the room through a door and cable holes, but the power supply was not cut off. An engineer warned that a tsunami could damage the generators. In response, TEPCO added doors to prevent water from entering generator rooms.
Five years before the accident, American nuclear scientists said manually operated venting systems were riskier than passive systems. Before its explosion, the venting system in unit 3 had several problems. By 2011, new reactor designs used passive venting systems.